Method and apparatus to remove shifting balls from frac sleeves in oil and gas wells

ABSTRACT

A process to drill through a spherical frac shifting ball in an oil well utilizes an improved mill, rotates drill pipe at one hundred to five hundred rpm, circulates drilling fluid such that the velocity of said fluid upwardly over said exterior of the drill pipe is in the range of three hundred to four hundred and seventy five feet per minute, and applies one thousand to three thousand pounds of slack off weight.

This application is a continuation-in-part of U.S. patent application Ser. No. 12/384,024 filed Mar. 31, 2009 which is a continuation-in-part of U.S. patent application Ser. No. 12/217,238 filed Jul. 2, 2008.

This invention pertains to a system to remove frac plugs in a well.

Oil or gas wells often have in the ground multiple formations. When there is a need to fracture individually these formations to stimulate them to better produce oil, temporary plugging agents or “frac plugs” are set at desired elevations in the well casing or bore to facilitate fracturing the formations in stages. After each desired formation has been fractured, the frac plugs are removed to enable operation of the well to produce oil or gas. Frac is a shorthand term for fracturing in connection with oil and gas wells.

Composite frac plugs are often utilized. These frac plugs include or incorporate a resin in combination with a ceramic, cloth, aluminum, cast iron and/or some other material. For example, one frac plug includes a resin body in combination with an aluminum mandrel and cast iron slips. Still another frac plug includes a resin body in combination with ceramic inserts. Some examples of commonly used composite frac plugs include the MILL EZ™ by Magnum Oil Tools, the SPEEDY LINE II™ by Halliburton, the QUICK DRILL 2™ by Baker Oil Tools, the PYTHON MT™ by BJ Services, the D2™ by Smith Services, and the FRACGUARD™ by Weatherford Completion Systems.

Conventional mills have long been utilized to remove frac plugs, as well as other materials including steel, cast iron, cement, dehydrated drilling mud, and dehydrated sand slurries.

I have discovered an improved process to remove materials from oil and gas well casings or bores.

This improved process is described with reference to the drawings, in which:

FIG. 1 is a top view illustrating a mill utilized in the system of the invention;

FIG. 2 is a section view of the mill of FIG. 1 illustrating additional construction details thereof;

FIG. 3 is a perspective view illustrating a tapered carbide insert that is welded onto a receiving seat in the mill of FIGS. 1 and 2;

FIG. 4 is an end view taken from the center 50 of the top of the mill of FIG. 1 illustrating a tapered carbide insert in position on a receiving seat in the mill prior to the insert being welded or otherwise secured to the seat;

FIG. 5 is a side view illustrating carbide inserts stacked in position on a pair of stepped receiving seats in the mill of FIGS. 1 and 2;

FIG. 6 is a top view illustrating the offset disposition of carbide inserts on succeeding seats in the mill of FIGS. 1 to 4;

FIG. 7 is a perspective view of a mill of the general type utilized in the method of the invention;

FIG. 8 is a side section view of the mill of FIG. 7 illustrating additional construction features thereof;

FIG. 9 is a side view illustrating a mill constructed in accordance with an alternate embodiment of the invention;

FIG. 10 is a section view illustrating further construction details of the mill of FIG. 9 and taken along section line 10-10 thereof;

FIG. 11 is a top view of the mill of FIG. 9 illustrating further construction details thereof;

FIG. 12 is a section view illustrating construction details of alternate embodiment of the mill of FIG. 9; and,

FIG. 13 is a diagram illustrating a spherical shifting ball in a frac sleeve.

Briefly, in accordance with the invention, I provide an improved process to drill through a composite frac plug in an oil well. The composite frac plug includes a resin in combination with at least one material selected from a group consisting of a ceramic, cast iron, aluminum and cloth. The process includes the steps of providing a mill including a plurality of spaced apart seats, each seat including an upstanding leg canted at an angle from the vertical in the range of eighteen to twenty-six degrees, and a plurality of carbide inserts affixed to each seat and including a peripheral edge extending outwardly from the seat; providing drill pipe having a distal end and a proximate end; attaching the mill to the distal end of the drill pipe; inserting the mill and the distal end of the drill pipe in the oil well until the mill contacts the top of the composite frac plug; rotating the mill at one hundred to five hundred rpm; circulating drilling fluid such that the velocity of said fluid upwardly over said exterior of said drill pipe is in the range of three hundred to four hundred and seventy five feet per minute; and, engaging the proximate end of the drill pipe and applying one thousand to three thousand pounds of slack off weight.

In another embodiment of the invention, provided is a process to drill through a frac plug in an oil well. The frac plug includes an exterior sleeve structure, and a spherical frac shifting ball mounted in the sleeve structure. The process includes the step of providing an elongate mill. The mill includes an elongate axis; an exterior; a distal end; a proximate end; a plurality of spaced apart primary rows of carbide inserts extending axially along the exterior from the proximate end to the distal end, and tapering inwardly at the distal end; and, a plurality of spaced secondary rows of carbide inserts, the secondary rows each intermediate and spaced apart from a pair of the primary rows of carbide inserts. The mill is shaped and dimensioned to drill the spherical frac shifting ball while leaving at least a portion of the exterior sleeve structure. The method also includes the steps of providing drill pipe having an exterior, a distal end and a proximate end; attaching the mill to the distal end of the drill pipe; inserting the mill and the distal end of the drill pipe in the oil well until the mill contacts the top of the frac plug; and, engaging the proximate end of the drill pipe and rotating the pipe and the mill at one hundred to five hundred rpm, circulating drilling fluid such that the velocity of the fluid upwardly over said exterior of the drill pipe is in the range of three hundred to four hundred and seventy five feet per minute, and applying one thousand to three thousand pounds of slack off weight.

Turning now to the drawings, which depict the presently preferred embodiments of the invention for the purpose of illustrating the practice thereof and not by way of limitation of the scope of the invention, and in which like reference characters refer to corresponding elements throughout the several views, FIGS. 1 to 2 illustrate a mill constructed in accordance with the invention and generally indicated by reference character 10. Mill 10 includes a plurality of spaced apart radial insert support structures 11 to 15. In use, mill 10 rotates in the direction indicated by arrow M in FIG. 1.

Insert support structure 11 includes flat (or if desired, convex or concave) ledge 26. A leg 11A outwardly depends from ledge 26 and includes outer edge 22, back surface 23, and front surface 24. When mill 10 is in the upright vertically oriented orientation illustrated in FIGS. 1 to 2, the leg 11A, 12A of each radial seat structure 11 to 15 is tilted or canted back at an angle Z (FIG. 4) from vertical axis X. Axis X is parallel to the axis of rotation of mill 10. Angle Z is one to three degrees, preferably about two degrees, from the vertical axis X. This angle Z is illustrated in FIG. 4 and is exaggerated in the drawings for purposes of illustration. Angle Z is important in the efficient operation of mill 10 because it functions to lessen initial cutting impact, friction and surface tensions, thereby increasing the operational life of the mill.

Inner seat 25 and outer seat 27 are each generally parallel to ledge 26, are each generally normal to surfaces 23 and 24, and each extend inwardly toward the center 50 of mill 10. As is more readily seen in FIG. 5, seat 35 is higher than, or is “stepped” up from, seat 27. Insert support structures 12 to 15 are each generally equivalent in structure to that of insert support structure 11. However, as can be seen in FIGS. 1 and 2, inner seat 35 associated with insert support structure 12 extends further inwardly than do the inner seats 25 associated with the other insert support structures 11, 13 to 15. In fact, inner seat 35 extends inwardly past the center 50 of mill 10.

Insert support structure 12 includes flat ledge 36. A leg 12A outwardly depends from ledge 36 and includes outer edge 32, front surface 33, and back surface 34. Seat 35 is generally perpendicular to surfaces 33 and 34 and, as noted, extends inwardly toward and past the center 50 of mill 10. The elevation of seat 37 is lower than the elevation of seat 35, just as the elevation of seat 27 is lower than the elevation of seat 25.

The stepped seats 25 and 27 of a insert support structure 11 receive a stacked pair of rows of conically shaped carbide inserts or cutters 40. Each insert or cutter 40 can, if desired, include one or more circular concave detents, or “chip breakers”, formed in the larger diameter face 42 of an insert or cutter 40 and within the outer circular peripheral edge 41 of the cutter 40. Each insert or cutter 40 is welded or otherwise secured to a seat 25 and 27 and any adjacent insert or cutter 40. Further, mill 10 is strengthened by welding or otherwise securing carbide particles 51 (FIG. 4) to the ledge 26 of the insert support structure 12. In addition, if carbide inserts 40 are worn or break and a leg 11A, 12A is worn away, carbide particles 51 function to cut and extend the life of mill 10. The leg 11A, 12A of each insert support structure 11 can also be strengthened by constructing mill 10 with a metal support that is positioned in the area normally occupied by particles 51. The metal support functions to thicken and strengthen each leg.

While the number of insert support structures 11 to 15 on a mill 10 can vary, five support structures are presently preferred as appearing to be most efficient in drilling a frac plug and/or other material. The ledge 26, 36 of each seat is currently preferably flat, and the ledge 36 of one insert support structure 12 extends past center on mill 10. An odd number of insert support structures 11 to 15 is preferred because an even number of support structures can produce harmonics that produce vibration and shaking and slow the cutting speed of mill 10. In use and testing, dimensional size limitations have, practically speaking, functioned to prevent the use of seven or more seats on mill 10.

A three and three-quarters inch O.D. mill has legs 11A, 12A with outer edges 22, 32 that are currently canted downwardly toward the center 50 at an angle Y (FIG. 2) of twenty-two degrees. The mill is made with two and three-eighths API regular pin up. The purpose of such canted edges 22, 32 is to cut material from the outside to the inside of the frac plug or other material being drilled with mill 10. Cutting the frac plug from the outside to the inside of the frac plug is believe to decrease the time required to drill the plug. The downward slope from the outer edge 22, 32 of each leg 11A, 12A, respectively, is indicated by angle Y (FIGS. 2 and 8) and is in the range of eighteen to twenty-six degrees, preferably twenty to twenty-four degrees, more preferably twenty-one to twenty-three degrees, and most preferably twenty-one and a half to twenty-two and a half degrees.

A four and five-eighths inch O.D. mill is made with two and seven-eighths API regular pin up. The downward slope from the outer edge 22, 32 of each leg 11A, 12A, respectively, of the four and five-eighths inch O.D. mill is indicated by angle Y and is in the range of eighteen to twenty-six degrees, preferably twenty to twenty-four degrees, more preferably twenty-one to twenty-three degrees, and most preferably twenty-one and a half to twenty-two and a half degrees. Angle Y is generally quite consistent regardless of the O.D. of the mill. Angle Y is presently twenty-two degrees, and as angle Y moves outside the range of twenty-one and a half to twenty-two and a half degrees the efficiency of the mill noticeably decreases, even though the invention can still be utilized at the angles noted outside the twenty-one and a half to twenty-two and a half range.

A six and one-eighth inch O.D. mill is currently made with two and seven-eighths API regular pin up. The downward slope from the outer edge 22, 32 of each leg 11A, 12A, respectively, of the six and one-eighth inch O.D. mill is indicated by angle Y and is in the range of eighteen to twenty-six degrees, preferably twenty to twenty-four degrees, more preferably twenty-one to twenty-three degrees, and most preferably twenty-one and a half to twenty-two and a half degrees.

The shape and dimension of the carbide inserts or cutters 40 (FIG. 3) can vary as desired, but are presently preferably are generally cylindrical with a three-eighth inch O.D. (outside diameter) at the larger diameter end, a five-sixteenths O.D. at the smaller diameter end, and a height typically in the range of three-sixteenths to one-quarter inch. The OD of the larger diameter end of a cutter 40 typically is in the range of one-eighth to three-quarters of an inch, with the smaller diameter end having an O.D. that is somewhat less. After a cutter 40 is affixed to an outer seat 25, 35 or is affixed to a cutter 40 on an inner seat 27, 37, a portion of the peripheral edge 41 extends outwardly and upwardly past the outer edge 22, 32 (FIG. 4) of a leg 11A, 12A.

Cutters 40 are presently preferably braised to a seat 25, 27, 35, 37 with nickel silver solder. The cutters 40 on a first seat pair 25, 27 are staggered, or offset, with respect to the cutters on the next succeeding seat pair 35, 37 (FIG. 6) such that the valleys or low areas between an adjacent pair of cutters on seat pair 25, 27 are offset from the valleys between an adjacent pair of cutters on seat pair 35, 37. This is accomplished by, as is illustrated in FIG. 6, beginning the row of cutters 40 on one seat pair 25, 27 with a full insert 40A and by beginning the row of inserts 40 on the next succeeding seat pair 35, 37 with a half of an insert 40B. Consequently, the inserts 40 along one seat pair 25, 27 include peaks that cut valleys in a frac plug and that leave raised areas intermediate the valleys. The inserts 40 on the next succeeding seat pair 35, 37 function to cut valleys in the raised areas left by the inserts 40 along seat pair 25, 27, and so on. As a result, offsetting the inserts 40 on a second succeeding seat pair 35, 37 from the inserts 40 on a preceding seat pair 25, 27 increases the cutting effectiveness of mill 10.

A mill 10 is presently preferably cast of steel or another desired material, but can be machined, can be assembled by welding together selected parts, or can be otherwise constructed.

One insert support structure 12 (and its associated inserts 40) preferably extends past the center 50 of mill 10 to provide cutting action at the center of mill 10. If each insert support structure met at, and did not extend past, the center 50, a grinding, instead of a cutting, action is produced.

In use of the method of the invention, a drill pipe is provided. The drill pipe has a proximate end and a distal end. The mill 10 is attached to the distal end of the drill pipe. The drill pipe and mill are inserted in the oil or gas well until the mill contacts the frac plug. A slack off weight in the range of 500 to 8,000 pounds is applied, preferably in the range of 2,000 to 3,000 pounds. The slack off weight is the total weight that is permitted to bear against the frac plug. The drill pipe it self may weigh weight 50,000 pounds, but most of this weight is supported by the drilling rig such that only 500 to 8,000 pounds bears against the frac plug. The mill 10 is then rotated at 100 to 500 rpm, preferably 120 to 500 rpm, and most preferably 140 to 500 rpm. The mill 10 can be rotated by rotating the drill pipe or by rotating mill 10 with a motor that is underground with mill 10.

Drilling fluid is pumped into the drill pipe, through the mill, and into the well casing, such that the velocity of fluid moving upwardly along the exterior of the drill pipe is in the range of 285 to 500 feet per minute, preferably 300 to 500 feet per minute. Drilling fluid can, by way of example and not limitation, comprise compressed air or salt water

One particular unexpected and unpredicted benefit discovered after the invention was developed is that the utilization of a higher RPM increased the speed with which a mill drills through a frac plug or other material.

Another unexpected and unpredicted benefit discovered after the invention was developed is that reducing the slack off weight to only 500 to 8,000 lbs, preferably 2,000 to 3,000 pounds, significantly increases the speed with which a mill drills through a frac plug or other material:

A further unanticipated benefit discovered after the invention was developed is that increasing the circulation velocity of drilling fluid significantly increases the speed with which a mill drills through a frac plug or other material.

Unless reasons exist to the contrary, judicial notice is taken of the following facts:

-   1. A dominant long felt trend currently exists in connection with     the drilling of frac plugs or other materials and teaches that the     typical RPM for a mill being utilized to drill out a frac plug or     other materials is sixty to eighty RPM. This trend has occurred over     an extended period of time, is followed by a large number of     individuals in the pertinent art, and likely can be demonstrated by     a significant number of references. A countervailing trend, if any,     is believed to be much weaker or to be obfuscated among other trends     in the art. -   2. A dominant long felt trend currently exists in connection with     the drilling of frac plugs or other materials and teaches that the     typical slack off weight for a mill being utilized to drill out a     frac plug or other materials typically is 10,000 to 12,000 pounds.     This trend has occurred over an extended period of time, is followed     by a large number of individuals in the pertinent art, and likely     can be demonstrated by a significant number of references. A     countervailing trend, if any, to utilize lower slack off weights is     believed to be much weaker or to be obfuscated among other trends in     the art. -   3. A dominant long felt trend currently exists in connection with     the drilling of frac plugs or other materials and teaches that the     upward velocity of drilling fluid is less than two hundred and     eighty-five feet per minute. This trend has occurred over an     extended period of time, is followed by a large number of     individuals in the pertinent art, and likely can be demonstrated by     a significant number of references. A countervailing trend, if any,     is believed to be much weaker or to be obfuscated among other trends     in the art. -   4. A commonly held belief in the oil and gas industry is that a     slack off weight in the range of 10,000 to 12,000 lbs ordinarily be     utilized when drilling a frac plug or other material. -   5. A commonly held belief in the oil and gas industry is that a mill     be rotated at sixty to eighty rpm when drilling a frac plug or other     material. -   6. A commonly held belief in the oil and gas industry is that the     velocity of drill fluid from the bottom of a well up ordinarily be     less than 285 feet per minute. -   7. There is no problem in the frac plug drilling art that provides     significant impetus for the development of the invention.     Conventional drilling methods have long been accepted. -   8. Making something better is a broad, general, long-existing     motivation that applies to each invention. Broad, general,     long-existing motivations likely provide little significant impetus     to produce an invention. For example, in the exercise machine art,     one broad, general, long-existing motivation is to make exercise     machines versatile, so that more than one exercise can be produced     on an exercise machine. This motivation may provide impetus to make     obvious modifications to a machine, but provides little significant     impetus to produce an invention. If, on the other hand, an exercise     machine produces a greater than normal number of injuries, such a     problem is more specific and provides strong impetus to improve the     machine. -   9. Key features of the mill 10 of the invention that improve the     efficiency with which the mill cuts include the extension of a bit     support structure 12 past the center of the mill, the concavity of     the mill, the particular angle of concavity Y, the number of bit     support structures, the use of round faced inserts 40, the use of     chip breakers in inserts 40, stacking rows of inserts 40 one on top     of the other, offsetting a row of insert 40 from the next successive     row in the manner depicted in FIG. 6, and rearwardly canting legs     11A, 12A through an angle Z in the manner depicted in FIG. 4. It is     a common belief in the oil and gas industry that the use of any one,     or a combination of two or more or all of said features do not     matter with respect to the drilling efficiency of a mill and will     not affect the drilling efficiency of the mill.

As noted above, oil or gas wells often are drilled in ground having multiple formations. When there is a need to fracture individually these formations to stimulate them to better produce oil, then temporary plugging agents or “frac plugs” are set at desired elevations in the well casing or bore to facilitate fracturing the formations in stages. After each desired formation has been fractured, the frac plugs can be removed to enable operation of the well to produce oil or gas. The process of installing (and then removing) individual frac, or “bridge”, plugs to isolate segments in a well is a relatively slow and expensive procedure. As a result, multistage frac sleeve “stimulation” systems were developed which provide logistical, safety, and economic advantages. These multistage systems can include twenty or more packer/trip pairs. Each pair is located adjacent a zone, or formation, which is intended to be “stimulated”, or fractured. The zones are stimulated sequentially. During a stimulation treatment, a sleeve 70 is opened by dropping a spherical frac shifting ball 71 (FIG. 13) located at the end of a frac stage. When a spherical frac shifting ball is dropped, the sleeve is opened to stimulate the next zone and to seal off lower zones.

Dropping a frac shifting ball 71 to open a sleeve 70 has been identified as one of the most economical methods of opening a sleeve. Multistage frac sleeve stimulation systems have used sequential sets of shifting balls in up to twelve or more zones. Each set of shifting balls is in one fourth inch increments. More recently, twenty zone systems utilize a set of shifting balls in one eighth inch increments.

One important requirement of multistage frac sleeve stimulation systems is that the frac shifting balls be impact resistant. The shifting balls must not shatter on impact with a frac sleeve. In one quality control test to evaluate the impact resistance of frac shifting balls, an air cannon fired frac shifting balls into the seat of a frac sleeve at a force simulating 68 bbl/min. This test replicated a ball drop during a high rate portion of a slickwater stimulation.

Another important requirement of a multistage frac sleeve stimulation system is that the shifting balls not become stuck in their seats in a sleeve.

The tolerance and strength requirements of frac shifting balls requires that they be made of a hard, relatively strong material. As a result, it can be difficult to mill the balls when they are being removed from a frac sleeve.

The mills illustrated in FIGS. 9 to 12 facilitate the removal of frac shifting balls. Although the mills in FIGS. 9 to 12 are in some respects similar to the mills illustrated in FIGS. 1 to 8, they also, of necessity, have significant structural differences.

The mill 52 illustrated in FIGS. 9 to 11 includes an elongate cylindrical body 59 with a centerline 72. A tapered conical tip 73 is attached to body 59 and comprises the distal end of mill 52. The exterior surface of mill 52 includes the cylindrical outer surface of body 59 and the conical outer surface of tip 73. If desired, additional carbide inserts 40A can be mounted on the distal, or outer, end of tip 73.

Primary rows 53, 54, 55 of carbide inserts or cutters 40A extend over the exterior surface of mill 52, including the portion of the exterior surface comprised by the conical outer surface of tip 73.

Secondary rows 56, 57, 58 of carbide inserts or cutters 40A extend over the portion of the exterior surface of mill 52 comprised by the exterior surface of cylindrical body 59. Secondary rows 56, 57, 58 presently preferably do not extend over the exterior surface of tip 73. Each secondary row 56 is spaced apart from and between a pair 53, 54 of primary rows.

Carbide inserts 40A have a generally cylindrical shape. The shape and dimension of inserts 40A can vary as desired. For example, conically shaped inserts 40 can be utilized in place of inserts 40A.

An elongate support leg 111A, comparable in shape and dimension to leg 11A, is connected to the exterior surface of body 59 and/or of tip 73. Each leg 111A provides support for inserts 40A that are mounted against the front face of leg 111A. Carbide particles (not shown) comparable to particles 51 (FIG. 4) can be welded or otherwise secured to a leg 111A or the exterior of body 59 adjacent the back face of legs 111A to strengthen mill 52 and the attachment of inserts 40A to the mill 52. The back face of leg 111A is spaced apart from and parallel to the front face of leg 111A. For purposes of comparison, in FIG. 1 surface 24 is the front face and surface 23 is the back face of leg 11A.

The primary rows 53 to 55 and secondary rows 56 to 58 are currently equally spaced about the perimeters of body 59 and tip 73. Consequently, arrows A extend through an angle of sixty degrees, as do each of arrows B, C, D, E, and F.

The shape and dimension of mill 52, including the spacing between the primary and secondary rows 53 to 58, can vary as desired. The length of distal end 73, indicated by arrows G, is presently five inches. The length of body 59, indicated by arrows H, is presently seven inches. The diameter of base 61 is presently two and a half inches. The length, indicated by arrows I, of base 61 can vary as desired, as can the length, indicated by arrows J, of tapered base member 76. Tapered base member 76 is, in one embodiment of the invention, externally threaded. The diameter, indicated by arrows L1 in FIG. 10, of body 59 is presently two inches. The diameter, indicated by arrows K, of the end of tip 73, is presently one inch. The angle, indicated by arrows L in FIG. 9, between the exterior surface of conical tip 73 and centerline 72 is in the range of one to twenty degrees, preferably four to seventeen degrees, and most preferably five to fifteen degrees. Legs 111A can be canted in the same manner as legs 11A, 12A.

As is indicated in FIG. 11 by arrows N, each primary row 53 to 55 angles laterally inwardly away from center line 72 as the primary row 53 to 55 nears the outer end of tip 73. Angle N can vary as desired, but is presently one to twenty degrees, preferably four to seventeen degrees, and most preferably five to fifteen degrees.

FIG. 12 illustrates an alternate embodiment of the invention comprising a mill 52A which is generally equivalent to mill 52 except that (1) body 59 is replaced by a body 59A having a diameter less than that of body 59, (2) the secondary rows 56A, 57A, 58A rows of inserts are secured near the exterior of body 59A, and (3) the primary rows 53A, 54A, and 55A of inserts 40A are attached on legs 62, 63, 64, respectively, which position the primary rows in a location with respect to base 61 which is comparable to that of primary rows 35, 54, 56 in FIG. 10 but which also positions primary rows 53A, 54A, 55A on a circular perimeter which has a diameter greater than the circular perimeter of body 59. Secondary rows 56A, 57A, 58A are mounted on a path comprising a portion of the circular perimeter of body 59. As a result, secondary rows 56A, 57A, 58A are inset with respect to primary rows 53A, 54A, 55A. Secondary rows 56A, 57A, 58A are each presently generally parallel to primary rows 53A, 54A, 55A. The amount by which the diameter of the exterior surface of body 59A differs from the diameter of base 61 can vary as desired in order to achieve insetting secondary rows 56A, 57A, 58A with respect to primary rows 53A, 54A, 55A.

In use of the method of the invention, a drill pipe is provided. The drill pipe has a proximate end and a distal end. The mill 52 is attached to the distal end of the drill pipe. The drill pipe and mill 52 are inserted in the oil or gas well until the mill contacts a frac sleeve or shifting ball in the frac sleeve. A slack off weight in the range of 500 to 8,000 pounds is applied, preferably in the range of 2,000 to 3,000 pounds. The slack off weight is the total weight that is permitted to bear against the shifting ball or other components in the frac sleeve. The drill pipe it self may weigh weight 50,000 pounds, but most of this weight is supported by the drilling rig such that only 500 to 8,000 pounds bears against the frac sleeve or shifting ball in the frac sleeve. The mill 52 is then rotated at 100 to 500 rpm, preferably 120 to 500 rpm, and most preferably 140 to 500 rpm. The mill 52 can be rotated by rotating the drill pipe or by rotating mill 52 with a motor that is underground with mill 52.

Drilling fluid is pumped into the drill pipe, through the mill 52, and into the well casing, such that the velocity of fluid moving upwardly along the exterior of the drill pipe is in the range of 285 to 500 feet per minute, preferably 300 to 500 feet per minute. Although not visible in FIGS. 9 to 12, mill 52, 52A is hollow and includes, in conventional fashion, at least one passage through which drilling fluid can flow through mill 52, 52A and out the tip 73. Drilling fluid can, by way of example and not limitation, comprise compressed air or salt water.

One particular advantage of the mill 52 of the invention is that the tapered tip 73 initially contacts and concentrates milling force on only a portion of the upper semi-spherical surface area of a shifting ball in a frac sleeve. Then, as the tip of mill 52 begin to penetrate the shifting ball, the primary and secondary rows of carbide inserts cuts through the shifting ball from the inside out.

Having described the invention in such terms as to enable those of skill in the art to make and practice it, and having described the presently preferred embodiments thereof, 

1. A process to drill through a frac plug in an oil well, the frac plug including an exterior sleeve structure, and a spherical frac shifting ball mounted in the sleeve structure, the process including the steps of (a) providing an elongate mill including (i) an elongate axis, (ii) an exterior, (iii) a distal end, (iv) a proximate end, (v) a plurality of spaced apart primary rows of carbide inserts extending axially along said exterior from said proximate end to said distal end, and tapering inwardly at said distal end, (vi) a plurality of spaced secondary rows of carbide inserts, said secondary rows each intermediate and spaced apart from a pair of said primary rows of carbide inserts, said mill shaped and dimensioned to drill said spherical frac shifting ball while leaving at least a portion of said exterior sleeve structure; (b) providing drill pipe having an exterior, a distal end and a proximate end; (c) attaching said mill to said distal end of said drill pipe; (d) inserting said mill and said distal end of said drill pipe in the oil well until said mill contacts the top of the frac plug; (e) engaging said proximate end of said drill pipe and (i) rotating said pipe and said mill at one hundred to five hundred rpm, (iii) circulating drilling fluid such that the velocity of said fluid upwardly over said exterior of said drill pipe is in the range of three hundred to four hundred and seventy five feet per minute, and (iii) applying one thousand to three thousand pounds of slack off weight. 